In recent years it has been established by both laboratory and by production scale experience that certain new atmospheres for the handling of molten magnesium and its alloys are superior to the SO.sub.2 inhibited atmospheres as well as the flux techniques which have been in commercial use for many years. Foremost among these new protective atmospheres are an atmosphere of air inhibited by a small concentration of SF.sub.6 or an atmosphere of substantially CO.sub.2 plus a small concentration of SF.sub.6 which is needed to inhibit the oxidation due to the presence of residual air or leakage air in the system. In production scale operations of handling molten magnesium, it is generally impractical to completely eliminate air.
Air plus SF.sub.6 inhibitor (air/SF.sub.6) is the simplest solution to the problem and is in widespread use particularly for holding pots for magnesium diecasting in which temperatures do not exceed about 1250.degree. F (677.degree. C). A CO.sub.2 atmosphere has the advantage that the resultant protective film on the surface of the melt is very thin but more importantly that it is effective to much higher temperatures of about 1600.degree. F (871.degree. C) or even beyond. Consequently, the CO.sub.2 /SF.sub.6 atmosphere is of more general usefulness, being applicable also to operations of alloying, refining, superheating, and sand casting, etc., which involve higher temperatures.
In current practice when using an air/SF.sub.6 atmosphere, since air does not have to be removed from the vessel, only a very small controlled flow of SF.sub.6 gas from the supply tank is necessary to maintain the desired concentration of a few tenths percent SF.sub.6 at the melt surface. On the other hand, when using a CO.sub.2 /SF.sub.6 atmosphere in accordance with current practice, the gas flow from the two supply tanks must be sufficient to substantially purge the air from the system. The magnitude of the flow will depend upon the size and configuration of the vessel and the size and shape of the various orifices in the cover of the vessel. The required proportions of CO.sub.2 and SF.sub.6 in the flow depend upon various factors such as the percent of residual air in the vessel and the temperature of the operations, the required SF.sub.6 concentration being higher or higher temperature or more air. Thus for certain production operations the concentration of SF.sub.6 in the CO.sub.2 flow may be maintained at 0.2% by volume, while for other production operations the concentration of SF.sub.6 in the flow may be as high as 3.0% by volume.
In any case attainment, by currently used techniques, of the optimum atmosphere in the vessel requires a connection to the high pressure tank supply and control of the composition by use of valves and gauges. For large stationary installations such an inflexible connection presents no problem, but for transportable vessels the necessity for piping to the gas supply tanks is a source of considerable inconvenience. Some industrial installations go to the extreme of carrying the gas tanks along with the transportable vessel. Obviously, for smaller crucibles such as are transported around a foundry, the problem has been most difficult. The thin protective film which forms on the melt surface is not permanent, but on the contrary loses its protective nature within a few minutes after removal from contact with the protective atmosphere, so that the gas shield must be constantly maintained.